Abstract

Meteorites preserve a wide range of oxygen isotopic signatures from the time of the Solar System's formation. Most of these rocks record complex histories, each phase of which has the potential for overwriting initial oxygen signatures. The unequilibrated ordinary chondrites reveal evidence of hydrothermal alteration through isotopic disequilibrium within chondrules and in secondary magnetites, which we can now constrain to temperatures of 140-180°C. The effects of this alteration are progressively obliterated by later thermal metamorphism. Further heating leads to melting (shown in achondritic meteorites), producing well-defined mass-fractionation lines using high-precision analyses. The oxygen from low-temperature minerals in carbonaceous chondrites reveals high levels of isotopic uniformity, suggesting that the aqueous alteration occurred under open-system conditions. The initial isotopic composition of the water from the ordinary chondrites is quite distinct from that in the carbonaceous chondrites, but both fall on a single line of slope 1.0, as do the initial anhydrous silicate compositions. This is taken to show that a process generating a mass-independent fractionation was responsible for most of the oxygen-isotopic variation seen in meteorites. Subsequent aqueous alteration of the meteorite parent bodies involving these components is then capable of producing the full observed variation.